I’ve become intrigued by the news reports about a disabled American spy satellite that’s destined to hit the Earth. If you’ve read any reports about this, the conventional wisdom is that an experimental imagery satellite that was built by Lockheed Martin and launched from Vandenberg Air Force Base in Dec. 2006, is set to hit the Earth any day now.
The classified spy satellite lost contact with ground controllers after reaching its designated low earth orbit (LEO), and its orbit has been steadily decaying ever since. Without a means to communicate and control the satellite, it is destined for an uncontrolled entry that could happen anywhere within a matter of weeks or months.
There have been some rather alarmist postings about the potential harm of the satellites impact, and discussion of potential environmental concerns. I decided to reach out to experts at AGI, the makers of the Satellite Tool Kit (STK), in order to understand how software tools can be applied to the problem, and to get a better sense of how this moving target might be tracked.
I spoke with Robert Hall, a product manager at AGI, who has experience with controlled reentry, having consulted with the Russian Space Agency on the reentry of the Mir Space Station in March 2001. The Mir Space Station was in direct contact with ground control and thrusters on the craft helped guide it toward reentry in the South Pacific Ocean.
While details aren’t publicly known about the size and orbit of the latest spy craft, it can be assumed to be much smaller than Mir, and its reentry will not be controlled. Hall made it clear that while tracking tools lend a great deal of insight into LEO satellite paths and eventual impact, the timing and location of reentry for an object that you don’t control is very hard to predict far in advance.
While the Air Force Space Command regularly monitors and maintains a catalog of space debris and items in orbit, the accuracy of those calculations is of limited usefulness for long-term predictions. Much more accuracy is possible with craft that you have control over, as direct and frequent contact with the craft from ground control helps to determine position, and many modern craft are also equipped with GPS sensors on board.
Given a known satellite location, AGI’s satellite software tools are very useful to propagate the orbit of a satellite forward to understand where it will be at a point in time. LEO satellites typically circle the globe 15 or 16 times per day. The accuracy in a short-term time window of a day or a few days can be close, but predicting further out is very difficult.
There are many forces acting on an LEO satellite. Atmospheric drag is constantly working to slow these satellites down and lower their orbit. This requires operators to periodically burn fuel to maintain the desired orbit. Without communication or fuel, the decay of this satellite will be impacted by solar activity that cannot be corrected. Predicting this process accurately over weeks and months can be difficult, in part because the atmosphere changes over time, reacting to things like solar activity.
The descent of anything into Earth’s atmosphere is very difficult to predict as it is not a linear process. When the craft gets low enough in its descent, the decay of the orbit becomes very steep and is variable based on the thickness and resistance of the atmosphere. There’s also the orientation of the satellite that will affect the drag on the craft as it enters. The size and shape of the craft is certainly not a symmetrical ball like Sputnik, so the force of resistance will vary depending upon how it’s positioned on reentry, and this will affect the trajectory.
There have been many satellite reentries, both controlled and uncontrolled, without any harm to humans. The Space Shuttle Columbia is one example to put things in perspective. This very large craft broke up at the altitude of about 210,000 feet over Texas and Arkansas, and no one was directly harmed or impacted by the debris.
There has been some talk about shooting this object out of the sky, while the option to take this space-based action is possible, it’s not politically or scientifically practical because of space debris. The Chinese tested the capability of shooting a satellite in space successfully a little more than a year ago. The outcome caused a debris problem that continues to endanger other satellites.
We’ll all watch the news for this latest satellite reentry with interest. I’m convinced that the probability is very low for direct harm to humans, and I’m certain all means of technology will be applied to safely direct it back to Earth.
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Where can I get tracking data to tell me when the satellite will be visable over Houston TX?
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